This application relates to ferroelectric thin films which are used in nonvolatile memories and specifically to a shallow junction metal-ferroelectric-metal-silicon semi-conductor. Known ferroelectric random access memories (FRAM) are constructed with one transistor (1T) and one capacitor (1C). The capacitor is generally made by sandwiching a thin ferroelectric film between two conductive electrodes, which electrodes are usually made of platinum. The circuit configuration and the read/write sequence of this type of memory are similar to that of conventional dynamic random access memories (DRAM), except that no data refreshing is necessary in a FRAM. Known FRAMs have a fatigue problem that has been observed in the ferroelectric capacitor, which is one of the major obstacles that limit the viable commercial use of such memories. The fatigue is the result of a decrease in the switchable polarization (stored nonvolatile charge) that occurs with an increased number of switching cycles. As used in this case, "switching cycles" refers to the sum of reading and writing pulses in the memory.
Another known use of ferroelectric thin films in memory applications is to form a ferroelectric-gate-controlled field effect transistor (FET) by depositing the ferroelectric thin film directly onto the gate area of the FET. Such ferroelectric-gate controlled devices have been known for some time and include devices known as metal-ferroelectric-silicon (MFS) FETs. FRAMs incorporating the MFS FET structure have two major advantages over the transistor-capacitor configuration: (1) The MFS FET occupies less surface area, and (2) provides a non-destructive readout (NDR). The latter feature enables a MFS FET device to be read thousands of times without switching the ferroelectric polarization. Fatigue, therefore, is not a significant concern when using MFS FET devices. Various forms of MFS FET structures may be constructed, such as metal ferroelectric insulators silicon (MFIS) FET, metal ferroelectric metal silicon (MFMS) FET, and metal ferroelectric metal oxide silicon (MFMOS) FET.
There are a number of problems that must be overcome in order to fabricate an efficient MFS FET device. The first problem is that it is difficult to form an acceptable crystalline ferroelectric thin film directly on silicon. Such structure is shown in U.S. Pat. No. 3,832,700. Additionally, it is very difficult to have a clean interface between the ferroelectric material and the silicon. Further, there is a problem retaining an adequate charge in the ferroelectric material. A FEM structure on a gate region is shown in U.S. Pat. No. 5,303,182, which emphasizes that the transfer of metal ions into the gate region is undesirable. Similar structure is shown in U.S. Pat. No. 5,416,735.
It is an object of this invention to overcome the aforementioned problems.
Another object of the invention is to provide an MFS FET device which provides a non-destructive readout.
Yet another object of the invention to provide an MFS FET device that occupies a relatively small surface area.
A further object of the invention is to provide an MFS FET device which requires a relatively low programming voltage.
Yet another object of the invention is to provide an FEM gate unit which overlies a shallow junction p.sup.- layer, which p.sup.- layer extends beyond the margins of the FEM gate unit.
The method of forming the FEM cell semi-conductor structure of the invention includes forming a device area for the ferroelectric memory (FEM) gate unit on a silicon substrate. Appropriate impurities are implanted into the device area to form conductive channels, for use as a source junction region, a gate junction region and a drain junction region. A FEM cell includes a FEM gate unit formed on the substrate. A gate junction region is formed between the source junction region and the drain junction region for the FEM gate unit on the FEM gate unit device area, which FEM gate unit includes a lower metal layer, a ferroelectric (FE) layer, and an upper metal layer. A shallow junction layer is formed between the FEM gate unit and the gate junction region, as another conductive channel. The FEM gate unit is spaced apart from the source region and the drain region, as is the conductive channel between the FEM gate unit and the gate junction region. Formation of the various conductive channels may take place at various stages of the manufacture, depending on what other devices are built on the substrate, and depending on the efficiencies of the various orders of construction.
The structure of the FEM cell semiconductor includes a substrate, which may be a bulk silicon substrate or an SOI-type substrate. Conductive channels of a first and a second type are located above the substrate. A FEM gate unit is located above a channel region, wherein the FEM gate unit includes a lower metal layer, an FE layer, and an upper metal layer. A conductive channel of a third type is located between the FEM gate unit and the channel region. The FEM cell may be constructed in series with a conventional MOS transistor.
These and other objects and advantages of the invention will become more fully apparent as the description which follows is read in conjunction with the drawings.